The invention relates to excimer and molecular fluorine lasers, and particularly to an apparatus and method for monitoring the energy of output laser pulses in a feedback arrangement and providing enhanced energy stability.
For many industrial and laboratory applications, excimer lasers are used in an operating mode wherein active stabilization of the output power of the laser is important. The active stabilization of the output energy of excimer lasers typically involves an energy detector indirectly connected to a control component of the high voltage drive for the discharge in the laser tube and, accordingly, actively adjusting the drive voltage to stabilize the output energy. This is possible because, as the output energy or output power of the excimer laser is selected to be maintained in a certain range, it is known that this output energy value depends upon the input high voltage drive, as is illustrated at FIG. 1. Thus, a variation of output energy may be compensated by adjusting the high voltage drive. See U.S. patent application Ser. Nos. 09/379,034, 60/123,928 and 60/124,785 (describing techniques for compensating output energy variation based on halogen depletion including gas replenishment, as well as high voltage adjustments over limited voltage ranges), each of which is assigned to the same assignee as the present invention and which is hereby incorporated by reference into the present application.
The present invention relates to the field of industrial excimer and molecular fluorine lasers and the application of these lasers in optical lithography, annealing, micro machining, and others. Excimer lasers used for these applications are mainly XeCl lasers (308 nm), KrF lasers (248 nm), and ArF lasers (193 mn). Molecular fluorine (F2) lasers (157 nm) are also used as well.
In these applications, optical imaging systems are used in combination with the laser. These imaging systems usually transform the output beam of the laser prior to illumination of a mask with the transformed laser beam. The mask is than imaged to the sample (wafer, workpiece) to form the desired light pattern for exposure.
A clear aperture is typically used in the optical imaging system causing the spatial beam size of the incident laser beam to be smaller after the aperture. The aperture improves homogeneity of illumination at the sample, as well as changes or varies the laser beam size, profile and/or divergence over the gas, laser tube, or other component lifetimes. For any particular application and a given sample or workpiece to be processed, the illumination energy density at the workpiece has been determined during process development or is determined with some calibration routines before or during processing.
For processing, the excimer or molecular fluorine laser is operated in an energy-stabilized or power-stabilized mode. An internal energy or power meter measures the output energy/power of the laser. A feedback circuit compares the actual energy/power with a desired, preset value. The high voltage from the laser power supply is set accordingly to an appropriate higher or lower value, depending on the result of the comparison. The internal energy/power meter module is usually designed to measure the total energy in the beam, and thus typically averages the spatial inhomogeneities of the beam profile (i.e., the spatial distribution of energy in the laser beam).
Typically, as illustrated at FIG. 2, a beam splitter 2 external to the laser resonator reflects a diagnostic beam portion 3 of the laser output beam 4 to the energy detector 6, e.g., a photodiode, photomultiplier tube, CCD array, PSD, pyroelectric sensor, etc. A working beam portion 8 of the output beam traverses the beam splitter 2 as it proceeds towards a workpiece. The energy detector 6 then integrates the energy or power of the entire diagnostic beam portion 3 split off by the beam splitter 2 to provide a measure for the total output energy or power of the working beam portion 8.
For industrial applications such as microlithography or TFT annealing, the excimer laser beam profile of the output beam at the workpiece/wafer will typically differ from the profile of the output beam at the point that it impinges upon the beam splitter. As illustrated at FIG. 3A, an optical component such as the input aperture 10 of an illumination system including a collimating lens 12 may be positioned along the optical path of the working beam portion 8 to produce a collimated beam 14 and to provide a selected beam profile.
FIG. 3B illustrates the beam profile 16 of the working excimer laser beam 8 before it traverses the lens aperture combination 12/10 of FIG. 3A. FIG. 3C illustrates the beam profile 18 of the collimated beam 14 that results from the working beam 8 traversing the lens aperture combination 12/10. The spatial homogeneity of the collimated beam 14 after the aperture 10 is improved over that of the beam 8 incident at the aperture 10. Referring back to FIG. 3A, the collimated beam 14 whose profile is shown at FIG. 3C then continues along the beam path towards beam shaping optics 20. Therefore, that portion of the energy of the incident beam 8 which is cut off at the aperture 10 never reaches the workpiece, and it is at the workpiece that the value of the energy dose is significant. Moreover, that portion of the incident beam that is not cut off at the aperture 10 and does traverse the lens 12 will undergo some induced absorption by the material of the lens 12, further differentiating the apertured and focused beam 14 from the incident beam 8.
FIGS. 4A-4B show alternative conventional optical arrangements 20 and 22, respectively, that the diagnostic beam portion 3 traverses before reaching the energy detector 6. The optical arrangement 20 of FIG. 4A includes an attenuator 24 for reducing the magnitude of the laser energy that will ultimately impinge upon the detector 6 in order to protect the detector 6, and a pair of diffuser plates 26, 28 disposed before the detector 6. As shown, the beam is so diffused after the second diffuser 28 that some fraction of beam energy escapes around the outside dimensions of the detector 6 and is not included in the measurement of the energy of the beam 3.
The optical arrangement 22 shown in FIG. 4B includes an attenuator 30 and a converging lens 32. In contrast with the arrangement shown at FIG. 4A, there is no portion of the beam 3 that is diffused outside of the dimensions of the detector 6. Some induced absorption, however, will occur at the lens 32 so the detector 6 will basically measure the energy of the split off beam portion 3 multiplied by the attenuation fraction of the attenuator 30, minus the induced absorption at the lens 32.
It is recognized that the measured energies at the detector 6 after the beam 3 traverses either of the arrangements 20, 22 shown in FIGS. 4A and 4B, respectively, are inaccurate measures of the actual energy dose that is incident at the workpiece. Further, the beam profile at the detector 6 does not match the beam profile incident at the work piece. For example, referring to FIG. 3A, the effects of blocking the outer portion of beam 8 by the aperture 10 and the induced absorption by the lens 12 are not measured by the detector 6 in conventional energy monitoring configurations. It is also recognized that it is the energy delivered to the workpiece, and not the energy of the beam 8 just after the beam 4 traverses the beam splitter 2, that should be stabilized by the feedback arrangement. Thus, it is desired to have an energy monitoring apparatus that achieves a more accurate measure of the energy and profile of the beam that is incident at the workpiece, so that the energy stability of the portion of the beam that is applied to the workpiece may be improved.
It is an object of the invention to provide an energy stabilization system for a working beam of an excimer or molecular fluorine laser wherein the energy of the beam at a workpiece exhibits improved energy stability.
It is a further object of the invention to provide an energy stabilization system for an excimer or molecular fluorine laser used for applications in which only a part of the spatial profile of the output beam from the laser resonator is applied to the workpiece.
In accord with the above objects, the present invention provides a laser system that includes a gain medium disposed in a resonant cavity, a power supply for exciting the gain medium to produce an output beam, beam splitting means for creating a primary output beam and a diagnostic beam from the output beam, beam transforming means for inducing a first beam parameter transformation in the primary output beam, beam simulation means for inducing a second beam parameter transformation in the diagnostic beam, and a detector for measuring at least one of a plurality of beam parameters of the diagnostic beam after the second beam parameter transformation is induced. The first beam parameter transformation induced in the primary output beam is substantially the same as the second beam parameter transformation induced in the diagnostic beam.
In another aspect of the present invention, the laser system includes a gain medium disposed in a resonant cavity, a power supply for exciting the gain medium to produce an output beam, beam splitting means for creating a primary output beam and a diagnostic beam from the output beam, beam transforming means for inducing a beam parameter transformation in the primary output beam, a detector for measuring at least one of a plurality of beam parameters of the diagnostic beam, and for producing a signal in response to the measured at least one beam parameter, and a processor for modifying the signal to simulate an inducement of the beam parameter transformation in the diagnostic beam.
The invention has the advantage that the energy in that part of the spatial profile of the working beam which is going to be used for application is stabilized. This is specifically advantageous for optical imaging systems where lens apertures in the optical system are xe2x80x9coverfilledxe2x80x9d by the beam (see FIG. 3A). The detector and feedback systems are in a preferred embodiment of the invention fast enough to compensate for variations of the energy/power density in the spatial part of the beam profile which is transmitted through the optical imaging system on a time scale between consecutive laser pulses.